1. Field of the Invention
The present invention relates to gate valves, and more particularly to self-relieving gate valves which possess floating seal arrangements to insure permanent and durable sealing engagement with the movable gate. The gate valve is of the slab type wherein the gate is operable within a hollow chamber to seal off a communicating fluid passage. The valve utilizes a pair of self-relieving sealing elements which are mounted in self-aligning and unrestrained relation in pockets in the fluid passage facing the gate for limited movement therewithin, and having annular gaskets mounted on the floating seal elements disposed in sealing contact with the gate and the mounting pockets.
2. Description of the Prior Art
Previously, most types of gate valves have utilized sealing elements which are press fit into pockets in the fluid passage adjacent the gate element, the sealing elements having an annular gasket facing both sides of the gate wherein the sealing elements are permanently and rigidly affixed within the pockets. Transverse movement of the gate element on opening and closing the valve frequently results in uneven wear of the gasket members because it is difficult to both initially align and maintain the sealing elements in perfect alignment with the facing surfaces of the gate. Any misalignment results in uneven wear of one or both of the sealing gaskets which ultimately results in leakage of the valve, either at the gate or at the mounting pocket wherein the fixed sealing element is disposed. It is common for the sealing elements to be rigidly attached to the valve body either by a high-pressure force fit or being welded in place for permanent attachment to the body. Such valve constructions are not self-relieving, can result in undue wear and ultimate valve failure at the seals where the gate may be operated virtually thousands of times during its useful working life.
There are various types of valves in existence of the floating seat type; however, most such structures utilize a single floating seat on one side of the gate while the other seat is permanently affixed to the valve body, the single floating seat normally being disposed on the upstream side of the valve.
There are in existence several types of gate valves which utilize floating seats which attempt to completely seal off the juncture between the valve seat pocket and the sealing element facing the gate with various types of elastomeric materials, or where the backside of the seat element is press fit into the valve body. Such valve seats are rigidly attached to the valve seat pocket to prevent leakage from the valve passage around its outside or the outside diameter of the seating element. It has been common practice to place a sealing element between the outside diameter of the sealing element and the internal diameter of the seat pocket, such sealing element normally being placed in the axial passage between the seal element and its receiving pocket. In such structures with the seal at that juncture, the seat element is permitted to float back and forth and remain in sealing contact with the valve seat so that no fluid passage occurs between such members. However, in such structures the problem which is thereby created, primarily associated with reverse acting valves, is that valves which close with their valve stem extending in an upward direction, i.e. typically those used for well head safety equipment as opposed to the direct acting valve which has a closed position with the valve stem facing downwardly, is that upon action of the valve body, pressure within the valve bore acting in concert with the stem, the upper portion of the stem being exposed to atmospheric pressure and the lower portion being located interiorly of the valve body, creates an unbalanced condition. Where the valve body is then internally pressurized with the well head fluids, the unbalanced effect causes the stem to elevate due to upward net forces where the gate has a solid portion in its bottom half. Thus, it will then close the valve. Such valves are commonly termed inherently fail-safe valves, wherein if any undue or unexpected conditions occur within the system, an actuator normally positioned in connection with the stem powers the valve so that when control pressure is relieved from the actuator, pressure within the valve body will cause the valve to close into an inherently fail-safe condition.
Typically, valves having floating seat elements are primarly designed to seal a valve from the upstream side, i.e. the side from which the pressure is exerted. Where the sealing element is completely sealed off within the valve seat pocket and is allowed to float, pressure acting between seating element and the sealing element that seals off the gate creates a differential effect between the outside diameter of the seal element and the valve seat pocket and the lesser diameter seal that seals off the gate. This differential in area with the valve body pressure coming from the upstream direction forces the floating seal element up against the gate, thus creating a seal on the upstream side. Thus, the seal element is forced against the gate and the entire valve cavity is sealed off in addition to the flow line downstream of the valve. However, where you employ a floating seat element in a reverse acting valve, the upstream sealing element causes the valve cavity to lose pressure through the downstream sealing element and where the stem is rising, effecting an elevation of the gate. A volume equal to the gate portion is removed from the valve cavity. Thus, upon displacement of the gate due to the stroke of the stem, a volume is being extracted causing the valve cavity to increase in volume by the same amount. When liquid at high pressure, say 3,000-4,000 psi, the stem is displaced out of the top of the valve, thus increasing the valve cavity volume. Pressure within the valve cavity is decreased very rapidly with an upstream floating seal element that is sealed to the valve seat pocket permanently and as it is allowed to move back and forth, it then seals against the gate. However, in the case of the reverse acting valve, as the gate is elevated to close off the fluid passage and maintain the seal element against the gate, an upper portion of the stroke must be completed since the gate has not been completely elevated to permit a reliable seal. At that point the gate only begins to seal so that it must be stroked further to guarantee that the seal remains in place and is fully reliable. Where the valve cavity is increasing appreciably in volume, as stated, as the gate attempts to rise and the pressure is falling rapidly due to increased expansion of volume in the cavity, very frequently the gate is not elevated to its full uppermost position, thus not fully engaging the valve seat element at the gate to form a reliable seal. In the case of reverse acting valves such gates frequently do not fully close unless an auxiliary closing force is applied to the stem such as by a strong mechanical spring being mounted on top of the valve actuator. Whether the gate operates upwardly or downwardly, reversing the action of the gate and seat element to have the gate move upward to close the valve without the aforesaid problems occurring have continued to be deficiencies in existing floating seat type valves.